Malonodinitrile (malononitrile) is known to be obtained by reaction of cyanogen chloride and acetonitrile at a temperature of 500.degree. C. up to 1200.degree. C., see e.g. Dixon U.S. Pat. No. 2,553,406, German OS No. 2449013, German Pat. No. 1768154, German Pat. No. 1911174 (and Lussling U.S. Pat. No. 3,729,499 related to both German Pat. Nos. 1768154 and 1911174); Japanese published application No. 41(1966)-16506; Taguchi U.S. Pat. No. 3,417,126.
The reaction is generally carried out without catalyst, see the above mentioned Japanese application No. 41(1966)-16506 and Taguchi U.S. Pat. No. 3,417,126. Also hydrogen cyanide, if present in larger than catalyst amounts, is employed to increase the selectivity, see Japanese Pat. No. 49-4207.
The great disadvantage in the production of malonodinitrile is in the very difficult isolation of the malonodinitrile from the reaction gas.
Generally the reaction gas is quenched as quickly as possible and thereupon the malonodinitrile recovered by fractional distillation, see e.g. Krebaum U.S. Pat. No. 3,055,738. The quenching is also assisted by use of a helping liquid such as acetonitrile itself, which can be either water free or water containing, as well as also by water or the condensed malonodinitrile itself, see German Pat. No. 1768154 and related Lussling U.S. Pat. No. 3,729,499.
However, the malonodinitrile can only be recovered from these quench solutions with losses.
The cause of these losses is based on the low stability of the malonodinitrile which can be decomposed explosively under the influence of strong acids or alkalis as well as with long heating above 100.degree. C.
If there is employed water or water containing cooling agent then the aqueous hydrochloric acid solution formed thereby strongly saponifies the malonodinitrile formed. For this reason the reaction mixture during the cooling is simultaneously neutralized. The salt containing waste water formed in the process thereby, which contains besides highly toxic materials such as acetonitrile dissolved in water, unreacted cyanogen chloride, etc., is difficult to remove.
Also substituted malonodinitriles, such as, e.g. o-chlorobenzalmalonodinitrile are produced with a cooling liquid, in this case with o-chlorobenzaldehyde, into which the reaction gas of the reaction of acetonitrile and cyanogen chloride is led, see Rosin U.S. Pat. No. 3,549,684. Yield and purity of the product, however, are not stated.
Since the cooling medium for better distribution in the reaction gas and therewith to the quicker cooling is injected into the reaction gas there is the danger that in use of the condensed product solution itself there is the danger of the clogging of the nozzles by solid impurities present in the condensed product, such as, e.g. neutralization products.
Although the experts have known of the need of a better yield and an easier isolation they have always proceeded from the thoughts of a condensation of the entire gas mixture, possibly through use of a cooling liquid. This principle is also once more manifest in recent times in German Pat. No. 2449013, in which it is true that the cooling temperature is raised to at least 40.degree. C. The condensed product solution itself serves as cooling agent. Through this, the formation of resin by saponification of the malonodinitrile because of the hydrochloric acid present should be avoided. If both the yields and the operating times of the quench part and of the after connected distillation part are to be improved then the resin formation as such is not eliminated.
In all of these processes particularly disadvantageous is the fact that the thermally sensitive malonodinitrile because of its higher boiling point can only be recovered after the distillation of the twofold to fivefold excess acetonitrile. Therewith it is heated for a long time at elevated temperature, through which decomposition products can form because of the thermal load and the yield lowered.
The object of the present invention therefore is the development of an industrially simple process that leads to the increase in yield of malonodinitrile (malononitrile) from the reaction gas.